Fix problem assembling sources and docs of modules.

A recent commit added 'overwrite=true' to all usages of the Ant copy task.

This was enough to show up a bug in our dist.src and dist.docs tasks, as was
seen on the Windows build:

    Failed to copy:
     
C:\Users\scala\.m2\repository\org\scala-lang\modules\scala-xml_2.11.0-M5\1.0-RC4\scala-xml_2.11.0-M5-1.0-RC4-javadoc.jar
to
     
H:\jenkins\workspace\scala-nightly-windows\dists\scala-2.11.0-20131019-064627-8848f24161\doc\scala-devel-docs\C:\Users\scala\.m2\repository\org\scala-lang\modules\scala-xml_2.11.0-M5\1.0-RC4\scala-xml_2.11.0-M5-1.0-RC4-javadoc.jar
...

This commit uses a "flattening" copy to put the JARs directly into 
scala-devel-docs.

The Ant docs note [1] this gotcha:

> Note that some resources (for example the file resource) return
> absolute paths as names and the result of using them without
> using a nested mapper (or the flatten attribute) may not be what
> you expect.

These appear to be the only places we fell into the trap:

    % ack '<file\b' --xml
    build.xml
    1948:      <file file="${scala-xml-javadoc}"/>
    1949:      <file file="${scala-parser-combinators-javadoc}"/>
    1992:        <file-sets/>
    2007:      <file file="${scala-xml-sources}"/>
    2008:      <file file="${scala-parser-combinators-sources}"/>

[1] http://ant.apache.org/manual/Tasks/copy.html